Frequency comb source with large comb spacing

ABSTRACT

A frequency comb laser providing large comb spacing is disclosed. At least one embodiment includes a mode locked waveguide laser system. The mode locked waveguide laser includes a laser cavity having a waveguide, and a dispersion control unit (DCU) in the cavity. The DCU imparts an angular dispersion, group-velocity dispersion (GVD) and a spatial chirp to a beam propagating in the cavity. The DCU is capable of producing net GVD in a range from a positive value to a negative value. In some embodiments a tunable fiber frequency comb system configured as an optical frequency synthesizer is provided. In at least one embodiment a low phase noise micro-wave source may be implemented with a fiber comb laser having a comb spacing greater than about 1 GHz. The laser system is suitable for mass-producible fiber comb sources with large comb spacing and low noise. Applications include high-resolution spectroscopy.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of application Ser. No. 12/955,759,filed Nov. 29, 2010.

This application is related to U.S. patent application Ser. No.12/895,127, filed Sep. 30, 2010, and entitled “Optical signal processingwith modelocked lasers”.

This application is related to U.S. patent application Ser. No.12/630,550, filed Dec. 3, 2009, and entitled “Highly rare-earth-dopedoptical fibers for fiber lasers and amplifiers”.

This application is related to U.S. patent application Ser. No.12/399,435, filed Mar. 6, 2009, and entitled “Optical scanning andimaging systems based on dual pulsed laser systems”.

This application is related to U.S. patent application Ser. No.11/546,998, filed Oct. 13, 2006, and entitled “Laser based frequencystandards and their application”, now U.S. Pat. No. 7,809,222.

This application is related to U.S. patent application Ser. No.11/372,859, filed Mar. 10, 2006, and entitled “Pulsed laser sources”,now U.S. Pat. No. 7,649,915.

The disclosures of application Ser. Nos. 12/955,759, 12/895,127,12/630,550, 12/399,435, 11/546,998, and 11/372,859 are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to fiber comb sources and amplifiers with largecomb spacing allowing for repetition rate and carrier phase control aswell as their application in precision metrology.

BACKGROUND OF THE INVENTION

Fiber laser based comb sources are becoming increasingly the laser ofchoice for applications in precision spectroscopy. Many suchapplications demand large comb spacing on the order of a GHz or severalGHz, precision comb control via adjustable repetition rates, as well ascarrier phase control. Moreover, the timing jitter and the carrier phasenoise of such fiber combs should be minimal.

Many applications for comb sources have indeed been identified and cancomprise, for example, high precision frequency synthesis, all opticalclocks (as described in T. Udem et al., ‘Optical frequency metrology’,Nature, vol 416, pp. 233 (2002) and frequency rulers for spectrographcalibration (as described in C. H. Li et al., ‘A laser frequency combthat enables radial velocity measurements with a precision of 1 cm/s’,Nature, vol., 452, pp. 610 (2008). Other applications have beenidentified in I. Hartl and M. E. Fermann, Laser based frequencystandards and their applications, U.S. Pat. No. 7,809,222.

Another important application involves low phase noise micro-wave orradio-frequency sources for atomic frequency standards, radar and remotesensing as described in A. Bartels et al., Femtosecond-laser-basedsynthesis of ultrastable microwave signals from optical frequencyreferences, Optics Letters, Vol. 30, Issue 6, pp. 667-669 (2005).

Notwithstanding numerous practical advances in fiber comb technology,fiber comb sources with large comb spacing are still difficult tomanufacture and not readily applicable to mass production. Moreover, thegeneration of broad frequency spectra with widely spaced comb lines ischallenging. Development of practical comb sources for use in the mid-IRspectral region remains particularly difficult.

SUMMARY OF THE INVENTION

In one aspect the present invention is directed to mass-producible fibercomb sources with large comb spacing and low noise, and theirapplications. The comb sources are based on mode locked waveguideoscillators.

At least one embodiment includes a mode locked waveguide comb laserhaving a compact dispersion control unit (DCU) in the laser cavity. TheDCU may be capable of producing net GVD in a range from a positive valueto a negative value, and preferably provides continuous adjustment ofdispersion. In various implementations a DCU may provide much lower lossthan achievable with dispersion compensators based on fiber Bragggratings.

In various embodiments, the use of a single angularly dispersive opticalelement provides a very compact and easily adjustable dispersioncompensation element. The dispersive element can be incorporated into amode locked waveguide cavity, allowing for both repetition rate andcarrier phase control and low carrier phase noise. In addition, low-lossbandwidth control is incorporated which is beneficial for theconstruction of pulse sources with high pulse energies.

By selection of large values of negative dispersion inside a highrepetition rate mode locked waveguide cavity, good pulse stability isobtained.

Broadband super continuum generation is further facilitated with reducedpower requirement via amplitude modulation of the output of thewaveguide oscillator.

Difference frequency generation is also further facilitated with reducedpower requirement via amplitude modulation of the output of thewaveguide oscillator.

The individual comb lines are rapidly tunable by the implementation ofcavity length tuning. The fiber laser output can further be locked to asingle-frequency laser such as a quantum cascade laser to produce arapidly tunable optical frequency synthesizer operating in the midinfra-red spectral region. Rapidly tunable frequency synthesizers canfurther be used in absorption spectroscopy.

Large comb spacing provides for resolution of individual comb linesusing standard spectroscopic techniques and the implementation ofbroadband absorption spectroscopy.

Low phase noise micro-wave sources can be constructed using fiber comblasers with large comb spacing by locking the comb laser to a precisionoptical reference frequency using a micro-wave beat signal between theoptical reference and the comb laser. The beat signal frequency canfurther be mixed with the measured carrier envelope offset frequency ofthe comb laser to generate a secondary beat frequency which isindependent of the carrier envelope offset frequency fluctuations of thecomb laser. The noise properties of the micro-wave source can further beimproved by actively stabilizing the output power of the comb laser witha feedback circuit connected to the pump power of the comb laser.

Multi-heterodyne spectroscopy can be performed using a mode locked laserwith dual wavelength output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a mode locked waveguide comb laserhaving tunable comb spacing.

FIG. 1A is a schematic illustration of a compact dispersion controlelement to be used in conjunction with waveguides.

FIG. 1B is a plot illustrating dispersion of the compact element of FIG.1A.

FIG. 2 is a schematic illustration of the generic components of a lowloss dispersion control element to be used in conjunction withwaveguides.

FIG. 3 is a schematic illustration of a mode locked fiber comb lasercomprising a compact dispersion control element.

FIG. 4 is a schematic illustration of a mode locked fiber comb lasersystem with large comb spacing, including an external modulator forsuper continuum and difference frequency generation with reduced powerrequirements.

FIG. 5 is a schematic illustration of a repetition rate tunable lasersystem coherently coupled to a single-frequency laser.

FIG. 6 is a schematic illustration of a repetition rate tunable comblaser system for use in broad band precision spectroscopy.

FIG. 7 is a schematic representation of a low phase noise micro-wavesource based on a fiber laser with large comb spacing.

FIG. 8 is a schematic illustration of a multi-heterodyne spectroscopysystem using a mode locked laser operating simultaneously on twowavelengths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents an exemplary embodiment of a repetition rate tunablewaveguide laser 100 with large comb spacing for use in a waveguide lasersystem. Tunable waveguide 100 laser may also include both repetitionrate and carrier phase control, and a dispersion control unit (DCU). Inthis example, a Fabry-Perot cavity design with two cavity mirrors isshown, where the first cavity mirror (right side) is mounted on apiezoelectric transducer allowing for piston type of control. The othercavity mirror (left side) is used for output coupling, and in thisexample is adjoined to the waveguide. The first cavity mirror is alsopart of the dispersion control unit, which allows for setting of theoverall cavity group velocity dispersion, where the overall cavity groupvelocity dispersion can preferably be set continuously from negative topositive values. Carrier phase can be controlled, for example, bycontrolling the optical pump power to the waveguide laser. An opticalpump can be incorporated via appropriate beam splitters and is notseparately shown. In various embodiments other techniques forcontrolling carrier phase may be utilized alone or in combination withpump control. For example, U.S. Pat. No. 7,649,915, FIGS. 8-10 and theassociated text describe several techniques, including arrangementsproviding pressure and/or temperature control of intra-cavity elements.

FIG. 1A represents an exemplary embodiment of a compact dispersioncontrol sub-system to be used in conjunction with a waveguide. Such adispersion control unit (DCU), is provided, which may include a bulkoptic element 103 providing angular dispersion such as a bulkdiffraction grating, a prism or a grism. In one configuration the systemfurther comprises two lenses 101, 105 and a mirror 107. The lens 101 onthe left hand side approximately collimates the output of the waveguide(not shown). The bulk optical element, diffraction grating 103 in thisexample, then diffracts the blue and red spectral parts of thecollimated beam, 113 and 111 (short dash-long dash and dot)respectively, into different angular directions, and thus providesangular dispersion. The focusing lens 105 focuses the beam onto a mirror107, which redirects the beams back to the grating 103 and to thewaveguide input via the focusing lens 101. Because the beam beingdirected back to the waveguide has a spatial chirp, the beam reenteringthe waveguide has a limited bandwidth. However, such a bandwidthlimitation can be very helpful to enhance laser stability when usingsuch dispersion control as part of a mode locked waveguide laser cavity.

In the example of FIG. 1 a configuration is shown that produces negativegroup velocity dispersion, or simply negative dispersion. The groupvelocity dispersion introduced by this configuration is, to a firstorder approximation, determined by 1) the grating groove density and 2)by the separation of the back focal plane of the focusing lens 105 (thepoint 115 where the red and blue beams cross between the grating and thefocusing lens) and the grating plane. For simplicity we refer to thisdistance as the effective grating separation (EGS). Here we refer to anEGS>0 when the beam crossing point is on the right hand side of thegrating plane and an EGS<0 when the beam crossing point is on the lefthand side of the grating plane.

It can be shown that the approximate value of the introduced groupvelocity dispersion by the effective grating separation is then close tothe value of the group velocity dispersion introduced by a classicalgrating pair with the same separation. By adjusting this effectivegrating separation the dispersion can be adjusted from positive tonegative, where positive dispersion is obtained when EGS<0 and negativedispersion is obtained for EGS>0. However, the advantage of using asingle-grating (versus a classical grating pair) is that less loss isintroduced at a central wavelength which is very important in theconstruction of fiber frequency combs in high Q cavities. In turn, highQ cavities are required for low noise operation.

In this example, and as illustrated in FIG. 1B, a 600 l/mm grating isbeing used at a wavelength of 1050 nm, which produces a round-tripdispersion of around −7000 fs² for an effective grating separation of 5mm. Diffraction gratings can have a diffraction efficiency of 98-99%,accounting for reflection losses on the lenses, such an adjustabledispersion control element can have a loss of only 5-10%.

Even lower losses can be achieved when replacing the lenses with mirrorsor when replacing the diffraction grating with a prism made from amaterial such as ZnS. The equivalence of lenses and mirrors, andgratings and prisms for group velocity dispersion compensationapplications is known from classical optics and does not require anyfurther explanation.

Moreover, in some embodiments, instead of using two lenses only one lensor mirror in conjunction with an angular dispersion inducing element ora prism can be used to introduce a controlled amount of dispersion intoa cavity. In the simplest configuration, an angle cleaved or polishedwaveguide can be used as a prism in conjunction with one or two lensesor mirrors to introduce a controllable dispersion into a cavity.

Several components of a low loss dispersion control unit are furtherexemplified in FIG. 2. In this example, elements of a dispersion controlunit, shown at an output of the waveguide, include at least onecollimation or focusing optical element and a single optical elementproducing angular dispersion. Moreover, a spatial chirp may optionallybe introduced at the endface of the waveguide in retroreflection.Group-velocity dispersion may be introduced in the cavity with anysuitable optical structure producing angular dispersion, and can be aprism (as schematically illustrated), grating, or any suitablecombination of diffractive, reflective, or refractive structures in bulkand/or integrated configuration, configured in bulk optical materialand/or on or near a surface.

A dispersion control element as used in a compact frequency comb laserwith large comb spacing is further shown in FIG. 3. In an exemplaryembodiment a Yb doped fiber can be used. By way of example, U.S. patentapplication Ser. No. 12/630,550, entitled “Highly rare-earth-dopedoptical fibers for fiber lasers and amplifiers”, discloses somehigh-gain fiber laser and amplifier configurations suitable for GHzrepetition rate fiber lasers. This fiber can have a core diameter of 4μm and can be designed with an Yb doping level providing an absorptionof around 50 dB/cm at 976 nm. Such fibers can produce a gain of up to 5dB/cm at 1030 nm when pumped with a few hundred mW at 976 nm. The pumpside of the fiber can be directly coated with a dielectric coatingproviding almost 100% transmission at 976 nm and around 0.1-10%reflectivity at 1050 nm. The angle polished or angle cleaved side of thefiber can be anti-reflection coated. The saturable absorber mirror canbe designed to limit damage from laser Q-switching as a distributedstructure with enhanced two-photon absorption as disclosed in U.S. Pat.No. 6,956,887, ‘Resonant Fabry-Perot semiconductor saturable absorbersand two photon absorption power limiters’ to Jiang et al. Preferably asemiconductor based saturable absorber mirror is implemented, but acarbon nano-tube or graphene-based saturable absorber as well known inthe state of the art can also be used.

The lens 303 on the left-hand side collimates the output of the fiberand can have a focal length of 0.45 mm and the lens 305 on the righthand side focuses the output onto the saturable absorber mirror and canhave a focal length of around 1.12 mm. A quarter wave plate compensatesfor any possible polarization losses inside the cavity. When using lowstress fiber mounting, depolarization in the intra-cavity fiber can beeliminated and the quarter waveplate can also be omitted. Alternatively,as known in the art, the polarization inside the fiber can beappropriately adjusted by the application of external stresses.

A transmission grating with groove density of 1000 l/mm with adiffraction efficiency of >98% at 1050 nm can be used for dispersioncontrol. The lenses 303, 305 can have a combined double pass opticalpath length of 7 mm, whereas the grating can have a round-trip opticalpath length of 3 mm. In a Littrow configuration the grating produces adispersion of around −6700 fs² per mm effective grating separation.Hence an effective grating distance of only 0.5 mm is required tocompensate for the round trip dispersion of around 4 cm of silica fiberoperating at 1050 nm.

The typical free space round trip optical path length for an effectivegrating separation of 0.5 mm of the above example is thus calculated as15 mm. Assuming a fiber length of 5 mm, the total round trip opticalpath length can be 30 mm. Hence a mode locked fiber laser with arepetition rate up to around 10 GHz can be constructed in this way. Themode locked fiber laser further operates in the negative dispersionregime. The total round trip cavity loss can be as small as 10% orsmaller

One benefit of using a dispersion control element as shown is that theintra-cavity loss is low, producing a high Q cavity and low carrierphase noise, while providing for flexibility for adjustment. The carrierenvelope offset frequency can be easily adjusted by either tilting themirror containing the saturable absorber or by movement of one of thetwo lenses perpendicular to the beam direction in the plane of thedrawing of FIG. 1. Alternatively a slit or an edge may be moved alongthe same direction. The repetition rate can further be adjusted byapplying a piston-type movement to the saturable absorber mirror.Moreover, other means of repetition rate and carrier phase control canbe implemented as, for example, described in U.S. Pat. No. 7,649,915 toFermann et al., and U.S. Pat. No. 7,809,222 to Hartl et al.

A mode locked comb laser was constructed according to the schematicillustrations shown in FIGS. 1 and 3, where a repetition rate of 1.5 GHzwas obtained with a Yb fiber length of 4 cm, where the intra-cavity losswas around 30-40%. The system operated with an intra-cavity peak powerof around 75 W at modelocking threshold with a pulse width of 400 fs.The corresponding minimum amount of self-phase modulation to achievemodelocking was estimated as 0.06. The laser operated with anintra-cavity dispersion of around −2000 fs² and was generating solitonpulses. The corresponding fiber dispersion was around +3200 fs².Assuming the same self-phase modulation and peak power requirements at10 GHz, the required peak intra-cavity power at 10 GHz in such a laserfor stable modelocking is calculated as 600 W, corresponding to anaverage intra-cavity power of 2.4 W, which is achievable when pumpingwith single-mode diode laser pump sources. Generally, an overall cavitydispersion of −0.1FD<−FD<−10FD or preferably a range of −0.3FD<−FD<−6FDis desirable for the generation of stable soliton pulses at >1 GHzrepetition rates, were FD is the absolute value of the fiber dispersion.The lower limit (in absolute value) is governed by laser stabilityconsiderations, whereas the upper limit is determined by the objectiveto produce the shortest pulses possible for subsequent coherentsupercontinuum generation. For later reference we refer to these valuesas the soliton stability regime.

In addition to Yb doped fiber, other rare-earth doped fibers usingdopants such as Er, Er/Yb, Tm, Ho/Tm, Ho, Nd or Pr may also beincorporated for the construction of fiber frequency combs with largecomb spacing. Instead of a bulk optics arrangement as shown here fordispersion control, chirped mirrors, Gires Tournois mirrors as well asdispersive saturable absorber mirrors can also be used for providingadjustable amounts of dispersion into a cavity. Schemes for appropriatedispersion and carrier envelope offset frequency control at GHzrepetition rates were, for example, discussed in U.S. Pat. No.7,649,915. When using fibers with overall negative dispersion,dispersion compensation is not required and repetition rates>10 GHz canbe obtained with no or few free space optics parts.

In addition to being valuable in the design of high repetition ratelasers, the compact dispersion control unit from FIG. 1 can also be usedas a bandwidth-limiting device in the design of fiber lasers operatingin the positive dispersion regime and similariton fiber lasers as, forexample, disclosed in U.S. Pat. Nos. 6,885,683 and 7,782,910. Incontrast to an optical filter or a fiber Bragg grating, the dispersioncontrol unit from FIG. 1 can be used for providing a smoothly adjustablebandpass filter with zero or adjustable dispersion. Such an arrangementis advantageous for the generation of high energy pulses. Such abandpass filter can be incorporated at the end of a Fabry-Perot cavityor as part of a ring cavity or even more complex cavity designs usingappropriate optical components. Polarization maintaining designs arealso possible. In the case of a Fabry-Perot cavity, a cavity design asshown in FIG. 3 can be implemented. In the case of a ring cavity thecompact dispersion control unit could be incorporated via anappropriately inserted polarization beam splitter. Many cavity designsfor mode locked lasers or passively mode locked lasers are known in theart and the present design can be incorporated into any of those.

One issue with comb lasers having large comb spacings is the increasedaverage power requirement for nonlinear spectral or frequency broadeningthat arise when increasing the comb or mode locked laser repetition rateto the multi-GHz regime. A way to avoid such a limitation is shown inFIG. 4. Here an amplitude modulator is inserted after the frequency combsource to slowly amplitude modulate the comb laser output. For examplesuch an amplitude modulator can be operating at a frequency of 1 kHz andproduce a mark to space ratio of 1 to 10, i.e. a transmission openingfor around 100 μs and blocked transmission for around 900 μs. One ormore fiber amplifiers may be disposed downstream from the modulator. Thepump power requirements for a fiber power amplifier to generate pulseswith a certain peak power are thus reduced by about a factor of ten.After amplification the pulses can then be coupled into one or severalnonlinear frequency broadening stages that can comprise, for example,supercontinuum generation and difference frequency generation. Invarious embodiments a highly non-linear fiber (HNLF) may be utilized inone or more stages. Such arrangements were discussed in U.S. patentapplication Ser. No. 12/895,127 and other applications incorporated byreference herein. Due to the reduced average power requirements in thisimplementation thermal management of the whole assembly is also greatlysimplified. Any amplitude modulator can be implemented, particularlyuseful are electro-optic modulators that can further be controlled tocompensate for time-varying responses of the fiber power amplifier, i.e.by appropriately controlled modulation a square pulse can be generatedat the output of the power amplifier and ringing and overshoots in thefiber amplifier can be avoided. Such designs are well known inconventional fiber amplifier technology and need no further explanation.

By implementing a relatively low amplitude modulation frequency and arelative long opening window, the spectral bandwidth of the individualcomb lines is only minimally affected as the achievable individual combbandwidth is approximately the inverse of the transmission window. For a100 μs transmission window a comb line width of around 10 kHz can thusbe achieved. The repetition rate as well as the carrier envelope offsetfrequency of the comb laser can be controlled using, for example, atleast one cw reference laser. Such schemes are well known in the stateof the art.

Alternatively, an f-2 f interferometer (not shown) can be inserted via abeam splitter after the nonlinear frequency broadening stage. Thecarrier envelope offset frequency can thus be directly measured;moreover, the carrier envelope offset frequency can be controlled usingappropriate electronic feedback to the oscillator pump current or otherintra-cavity components as discussed with respect to FIG. 1. Low noisecomb lasers are beneficial for such schemes because they minimizecarrier phase fluctuations between separate temporal transmissionwindows of the amplitude modulator. Any suitable rare-earth amplifiercan be implemented in such schemes, moreover, solid-state, semiconductorbased or micro-ring resonator based multi-GHz oscillators can beimplemented here in conjunction with fiber amplifiers. Appropriatemicro-ring resonators were, for example, described in P. Del'Haye etal., ‘Optical frequency comb generation from a monolithicmicroresonator’, Nature, vol. 450, pp. 1214-1217 (2007). Such fiber combsources can be broadly applied as frequency rulers as, for example,required in the calibration of astronomical spectrographs.

Some applications may require even higher repetition rates and thesecan, for example, be produced by the incorporation of repetition ratemultiplication cavities, i.e. cavities that are configured with anoptical round-trip path length at precisely a harmonic of the oscillatorround-trip path length. Repetition rate multiplication with factors of2-100 can then be readily achieved as is well known in the state of theart and was, for example, suggested in T. Sizer in ‘Increase in laserrepetition rate by spectral selection’, IEEE J. Quantum Electronics,vol. 25, pp. 97-103 (1989) and U.S. patent application Ser. No.12/895,127 ‘Optical signal processing with modelocked lasers’, toFermann et al. and is not further described here.

A combination of the systems shown in FIGS. 3 and 4 allows theconstruction of rapidly tunable frequency synthesizers. Such a design isschematically shown in FIG. 5. Referring to FIG. 5, the synthesizersystem includes a tunable GHz comb laser 500 coupled to a combcontroller for repetition rate control. A tunable multi-GHz fiber combsystem can be constructed using a design as shown in FIG. 3 by locatingthe saturable absorber mirror onto a piezoelectric transducer (notshown) with piston control which modifies the repetition rate of thelaser. Due to the spatial chirp of the beam on the saturable absorbersuch a scheme can also produce some variations in carrier envelopeoffset frequency.

Alternatively, a mode locked fiber laser with repetition rate controland low variation in carrier envelope offset frequency can be derivedfrom the design shown in FIG. 3 by applying a piston type of control toa cavity mirror where there is no spatial chirp. For example the pumplight could be coupled into the doped optical fiber by removing thedichroic coating on the left hand side of the fiber as shown in FIG. 3and replacing it with a bulk optics dichroic beam splitter. The pumplight can then be coupled into the fiber via the beamsplitter and theintracavity light can be directed onto a separate mirror with pistoncontrol. Additional beam splitters could be inserted for output couplingor simply the residual zero order output of the intra-cavity gratingcould be used for output coupling. Other cavity designs were, forexample, discussed in FIGS. 17 a and 17 b of U.S. patent applicationSer. No. 12/895,127 ‘Optical signal processing with modelocked lasers’,to Fermann et al. and are not further explained here. Otherimplementations and variations are possible.

Referring again to FIG. 5, after the tunable comb generator 500, anoptional nonlinear spectral or frequency broadening stage (as discussedabove with respect to FIG. 4) is included for increased spectralcoverage. A single line from the broadband spectral output is then mixedwith the output of a wavelength tunable single-frequency slave laser,for example a quantum cascade laser. A quantum cascade laser may have anominal wavelength in the mid to far infrared IR wavelength range. Bydirecting the comb output and the quantum cascade laser output onto adetector and limiting the detection bandwidth of the detector using adetector/filter arrangement (e.g.: photodetector and RF filters) aparticular comb line can be selected as illustrated. Additional opticalfilters can also be implemented for comb line selection or to reduce theshot noise on the detector. RF and optical filtering techniques are wellknown in the state of the art and are not further explained here.Tunable single-frequency lasers derived from locking the opticalfrequency of the single-frequency laser to the output of a tunable comblasers were, for example, discussed by B. R. Washburn et al. in‘Fiber-laser-based frequency comb with a tunable repetition rate’, Opt.Expr, vol. 12. Pp. 4999 (2004). However, only a comb spacing of 50 MHzwas used.

The output from the detector/filter module may be utilized to controlthe slave laser in a feedback arrangement. The beat signal between thecontinuum output from the broadening stage and the slavedsingle-frequency laser, which may preferably be a quantum cascade laser,can then be used in the feedback loop to control the wavelength of thesingle frequency slave laser as illustrated in FIG. 5. Analog and/ordigital signal processing (not shown) may also be utilized in thefeedback system to monitor and/or control various parameters. Forexample, while tuning the repetition rate of the comb laser the outputwavelength of the quantum cascade slave laser can also be tuned bysimply keeping the beat signal frequency constant. This can be done, forexample, by appropriately controlling the drive current to the quantumcascade laser, or controlling its temperature, as well known in thestate of the art. Other schemes for wavelength tuning of quantum cascadelasers were, for example, discussed in S. Bartaline et al., ‘Frequencymetrology with quantum cascade lasers’, Proceedings of SPIE, Vol. 7222,pp. 72220C1-1-72220c1-10 and are not further discussed here.

One benefit of a fiber comb laser with large comb spacing is thatwavelength calibration is simplified because of the reduced number ofspectral lines; moreover the required piston movement δL for a certainfrequency shift δf is proportional to the square of the repetition rateof the comb laser, i.e. δf=cf²δL. The use of a low noise high repetitionrate comb laser allows for rapid frequency synthesis with a quantumcascade laser while minimizing the requirements for carrier envelopeoffset frequency stabilization of the comb laser. For applicationsrequiring only a moderate frequency resolution of 10-100 MHz, continuouscontrol of the carrier envelope offset frequency can even be eliminated.Comb laser repetition rates for frequency synthesis with quantum cascadelasers of >1 GHz are particularly desirable; for some applicationsrepetition rates>250 MHz can be sufficient. When using differencefrequency mixing in order to generate a spectral output in the vicinityof the quantum cascade laser, the carrier envelope offset frequency isset to zero as is well known in the state of the art and, for example,discussed in U.S. Pat. No. 6,724,788 to Holzwarth et al. This allows thedetermination of the optical frequency of the single-frequency laserfrom just the repetition rate of the comb laser as well as the frequencycomb order.

Frequency comb lasers with large comb spacing can further implemented inhigh resolution spectroscopy applications in combination withconventional spectroscopic elements as shown in FIG. 6. Here arepetition rate tunable fiber frequency comb laser with a comb spacing>1GHz based on rare-earth dopants such as Yb, Tm, Nd, Pr, Er, Er/Yb or Hoor Ho/Yb is illustrated. The comb laser can further include a means forcontrol of the repetition rate and carrier envelope offset frequency,for example as discussed with respect to FIG. 1 and FIG. 3. Further, asillustrated in FIG. 6, detector D1 receives outputs from a referencelaser and a tunable comb laser and derives a beat signal useful forfrequency monitoring, or for control of the tunable GHz laser via afeedback loop (not shown). Optical referencing can, for example,comprise interfering the comb with a fixed continuous wave referencelaser as also shown in FIG. 6 using two beamsplitters BS1 and BS2 aswell as detector D1 for detection of the beat signal. The observed beatfrequency is then directly proportional to the distance between a combline and the single frequency laser in frequency space.

As illustrated in FIG. 6, an optical frequency or spectral broadeningstage can be used, and may be disposed downstream of the amplitudemodulator. This spectral broadening stage can employ differencefrequency mixing in order to eliminate the uncertainty of the carrierenvelope offset frequency in the comb output. An additional amplitudemodulator can also be used as discussed with respect to FIG. 4 so as toreduce the average power requirements for nonlinear spectral broadening.Also, additional fiber amplifiers (not shown) can be implemented afterthe amplitude modulator as discussed with respect to FIG. 4.

The output from the pulse source then passes through a sample undertest, for example a gas cell, before being dispersed in one or twodimensions with one grating spectrometer and a virtually imaged phasearray (VIPA) as is well known in the state of the art. With sufficientcomb spacing the individual comb lines can then be resolved in one ortwo dimensions and imaged onto a one or two dimensional detector arrayrespectively. An optical resolution of around 1-10 GHz per pixel of thedetector is thus achievable.

A scheme with a solid-state laser based multi-GHz repetition rate combsystem and a two dimensional angular dispersion element as well as a twodimensional detector array was previously described in S. Diddams etal., ‘Molecular fingerprinting with the resolved modes of a femtosecondlaser frequency comb’, Nature, vol. 445, pp. 627 (2007). However, asystem with a fiber laser based multi-GHz repetition rate comb laser wasnot considered. With advancements as described herein low noisefrequency comb lasers operating at repetition rates of 10 GHz and highercan be constructed which make such schemes very attractive. Moreover, afrequency resolution equivalent to the comb line width can be obtainedby slowly scanning the repetition rate of the frequency comb laser whiledetecting the modulation of the signal on each individual pixel of thedetector array. For frequency comb spacings larger than around 10 GHz, atwo dimensional detector array is also not required as individual comblines can, for example, be resolved using two or more conventionaldiffraction gratings in series or multiple passes or reflections from asingle grating. The use of a single-dimensional detector array generallyreduces the cost of the detection system while increasing theacquisition speed.

In addition the position of the comb lines can be slowly scanned and atthe same time modulated at high frequencies in frequency space in orderto enable broad band differential absorption spectroscopy on multipleabsorption bands simultaneously. Such schemes are well known from singlelaser spectroscopy. Many other spectroscopic techniques can be adaptedto broad band detection where the principle requirement is the opticalresolution of individual comb lines.

The generic set-up of such detection schemes involves multi-comb linefrequency analysis, where a tunable fiber frequency comb generator isprovided, a sample is illuminated with the plurality of the comb lines,the individual comb lines are transmitted through- or reflected from thesample and are optically resolved and imaged onto a one or twodimensional detector array and a physical characteristic of the sampleis determined from the response of the detector array to the physicalcharacteristic of the sample.

The individual frequencies of the individual comb lines can bedetermined or controlled by measurements of the instantaneous repetitionrate of the frequency comb as well as the instantaneous carrier envelopeoffset frequency using, for example, an f-2 f interferometer (not shown)as well known in the state of the art. Other methods of determining theinstantaneous frequencies of the comb lines are also possible and can,for example, involve optical referencing. As discussed above, opticalreferencing can, for example, comprise interfering the comb with a fixedcontinuous wave reference laser as also shown in FIG. 6 using twobeamsplitters BS1 and BS2 as well as detector D1 for detection of thebeat signal. The observed beat frequency is then directly proportionalto the distance between a comb line and the single frequency laser infrequency space. Optical hybrids (as well known in the state of the art)can further be implemented to determine on which side of the cw laserthe comb line is located. More than one cw reference laser can furtherbe implemented to measure the absolute location of the scanning comb infrequency space. Optical referencing has the advantage that no precisecontrol of the actual carrier envelope offset frequency or repetitionrate of the tunable comb laser is required. Optical referencing was, forexample, discussed in U.S. patent application Ser. No. 12/895,127‘Optical signal processing with modelocked lasers’, to Fermann et al.Several other schemes for the measurement of the instantaneous opticalfrequency of frequency swept single-frequency lasers were discussed inF. R. Giorgetta et al., ‘Fast high-resolution spectroscopy of dynamiccontinuous-wave laser sources, Nature Photonics, (2010) and are notfurther discussed here. When applying these methods it is beneficial tofilter out one individual comb line as easily possible with widelyspaced comb lines, for example, by using fiber gratings.

Another attractive application of fiber frequency comb lasers with largecomb spacing is as low phase noise micro-wave sources. An exemplaryembodiment of a low phase noise fiber comb based micro-wave source isshown in FIG. 7. In the example of FIG. 7 optical paths are depictedwith dashed lines and electrical feedback signal paths with solid lines.

A fiber comb laser with a comb spacing>1 GHz is shown in FIG. 7. One ofthe comb lines from the comb laser is combined with the output of anoptical continuous wave reference laser and detected with detector D1 asearlier discussed, for example with respect to FIG. 5. Detector D1 ofFIG. 7 produces a corresponding first beat frequency, S1.

The carrier envelope offset frequency of the comb laser is also detectedwith the f-2 f interferometer (as well known in the state of the art).The carrier envelope offset frequency can be phase locked to an RFsource or be left free running as described in J. Millo et al.,‘Ultra-low-noise microwave extraction from fiber-based optical frequencycomb’, Opt. Lett., vol. 34, pp. 3707 (2009). As described in J. Millo etal., the measured carrier envelope offset frequency can further be mixedwith the first beat frequency to produce a secondary beat frequencywhich is independent of the carrier envelope offset frequencyfluctuations. FIG. 7 schematically illustrates a feedback arrangementwith feedback signals from both the f-2 f interferometer output (CEO)and detector D1 output (S1). RF mixer 705 provides an output at asecondary beat frequency S2 which is transmitted to the comb controlmodule, which is configured for at least repetition rate control of thecomb laser. The secondary beat frequency (S2) is then phase locked to alow noise RF reference signal (not shown), which in turn stabilizes therepetition rate of the comb laser independent of carrier envelope offsetfrequency fluctuations. As a result, a low phase noise micro-wave signalcan then be extracted via directing the optical output of the comb laseronto detector D2. Other modifications are also possible.

To further reduce the phase noise of the micro-wave source the amplitudefluctuations of the laser can be minimized by stabilizing the outputpower of the fiber comb laser via a secondary feedback loop connected tothe laser pump (not shown). Because of the large modulation bandwidthsof Yb and Tm fiber lasers, amplitude noise minimization via pump powercontrol can be much more effective compared to Er fiber lasers, as usedby J. Millo et al. Moreover, repetition rates>1 GHz are further verybeneficial for detecting a low phase noise micro-wave signal fromdetector D2 as they minimize shot noise on the detectors. Preferably fora 10 GHz micro-wave reference an optical comb laser with a 10 GHz combspacing is used, with about 1 GHz or greater being suitable.

Although the optical separation of frequency comb lines is attractivefor some applications, various other applications are better served bysimultaneous detection of all comb lines and distinguishing them by thebeat signals as, for example, done in multi-heterodyne spectroscopy asdescribed in U.S. patent application Ser. No. 12/895,127 ‘Optical signalprocessing with modelocked lasers’, to Fermann et al. A common designlimitation of such systems is the requirement for coherent pulse pairsthat slowly scan through each other thereby producing a pulse separationwhich varies as a function of time. Such coherent scanning delay linesare conveniently produced using, for example, two comb lasers operatingat slightly different repetition rates or alternatively using arepetition rate tunable comb laser in conjunction with imbalancedMach-Zehnder interferometers. Both systems are relatively complex andthe latter system further requires mechanical moving parts which are notpermissible in some applications.

On the other hand it has long been known, however, that only one lasercan be configured as a scanning delay line as, for example, described inU.S. Pat. No. 5,479,422: ‘Controllable dual-wavelength operation ofmodelocked lasers’ to Fermann et al. The comb laser system as describedwith respect to FIG. 3 hereof greatly simplifies the construction ofsuch scanning delay lines as explained with respect to FIG. 1. With anarrangement as shown in FIG. 1A, and after angular separation andfocusing of the beam diffracted by the intra-cavity grating, the red andblue parts of the optical pulse spectrum can be well separated on thesaturable absorber. Therefore dual wavelength operation can be inducedby simply incorporating an appropriate groove on the saturable absorbermirror or locating a thin wire in front of the saturable absorbermirror. Moreover, the repetition rates of each color can be separatelycontrolled by splitting the saturable absorber in half and putting oneof the two halves onto a piezo-electric controller which incorporatespiston movement.

An exemplary dual wavelength comb system is shown in FIG. 8. It is verysimilar to the design shown in FIG. 3, but it now includes the splitsaturable absorber mirror, where one half is stationary and the otherhalf is mounted on a piezo-electric transducer with piston type controlfor adjustment of the difference in repetition rates between the twowavelengths. The two wavelengths can further be separated by including athin wire inserted between the focusing lens in front of the saturableabsorber mirror and the saturable absorber mirror itself, where the wirestraddles the intersection between the moving and stationary mirror.

Other coherent scanning delay lines based on two wavelength lasers canalso be constructed, for example the angular dispersive components andother optical elements as shown in FIG. 1A can also be inserted in frontof a regular intra-cavity mirror that does not contain a saturableabsorber. Also conventional dispersion compensating elements such asbulk grating pairs can be used for wavelength separation. Further, asdescribed in U.S. Pat. No. 5,479,422 gain media with inhomogenousbroadening such a Nd are particularly attractive for dual wavelengthoperation as they minimize cross saturation effects. However, the designas shown in FIG. 8 is particularly attractive because of the lowcomponent count and can ensure long term stable operation.

Such dual wavelength lasers operating at slightly different repetitionrates can then be used for multi-heterodyne spectroscopy by implementingadditional spectral broadening stages after the oscillator which producespectral overlap between the outputs of the oscillators. Once spectraloverlap between the two combs exists, beat signals between comb pairsbelonging to the two different repetition rates can be detected and usedfor multi-heterodyne spectroscopy. Moreover, the two repetitions ratescan be locked to each other with minimal relative variations of therelative carrier envelope offset frequencies. Alternatively opticalreferencing can be implemented to precisely measure repetition ratevariations between the two combs. Such schemes were discussed forexample in U.S. patent application Ser. No. 12/895,127 ‘Optical signalprocessing with modelocked lasers’, to Fermann et al. and are notfurther explained here.

Thus, the inventors have disclosed an invention in which at least oneembodiment includes a mode locked waveguide laser system, including, forexample, a fiber laser. The waveguide laser includes a laser cavityhaving a waveguide. An intra-cavity beam is emitted from the waveguide.A dispersion control unit (DCU) is disposed in the cavity and in anoptical path of the intra-cavity beam. The laser cavity is configured insuch a way that an intra-cavity beam is redirected to the waveguideafter traversing at least the DCU. The DCU imparts angular dispersionand group-velocity dispersion (GVD) to the intra-cavity beam duringpropagation in the cavity. The DCU also imparts a spatial chirp to theredirected beam. The DCU is capable of producing net GVD in a range froma positive value to a negative value.

A mode locked waveguide laser may include a mode locked fiber laser.

The DCU may include a diffraction grating and a lens system.

The DCU may include a lens system, and may include a prism or grism.

The DCU may include one or more of a diffraction grating, prism, grism,and angled waveguide endface, and may include one or more of an opticallens and mirror.

A means for control of the carrier envelope offset frequency of thelaser may be included.

The means for carrier envelope offset frequency control may include anoptical element and a mechanism for translating the optical elementalong an axis. In some embodiments a pressure and/or temperature controlsystem for intra-cavity elements may be implemented with one or morefeedback loops.

The mode locked waveguide laser may be configured as a soliton laser.

The mode locked waveguide laser may operate at a repetition rate greaterthan about 1 GHz.

The mode locked waveguide laser system may be configured to operate atdual wavelengths simultaneously, and the DCU may be configured toprovide a wavelength separation of the dual wavelengths.

The waveguide laser may include a fiber laser, and may be configured toprovide a different repetition rate for each of the two wavelengths.

The mode locked laser system may be configured for multi-heterodynespectroscopy.

The mode locked waveguide laser may include a split mirror.

The mode locked waveguide laser system may include a spectral broadeningstage.

The mode locked waveguide laser system may include a repetition ratemultiplier.

The mode locked waveguide laser system may include: an optical modulatorproviding a pulse train with a mark/space ratio>2; at least one fiberamplifier; and one or more spectral broadening stages downstream fromthe mode locked waveguide laser.

The mode locked waveguide laser may be configured such that a temporalbandwidth of the mode locked waveguide laser is limited, at least inpart, by the spatial chirp.

The DCU may be configured to displace at least one optical element ofthe DCU so as to adjust the net GVD to a value within the range.

At least one embodiment includes a tunable fiber frequency comb systemconfigured as an optical frequency synthesizer. The tunable comb systemincludes a fiber comb laser and a comb controller to control the comblaser, and to provide tunable comb spacing. The system includes a singlefrequency laser, for example a quantum cascade laser, that generates anoptical output at substantially a single optical frequency correspondingto a wavelength in the mid to far IR wavelength range. A frequencybroadening stage receives an output of the fiber comb laser, and thefrequency broadening stage may be configured to produce spectral overlapwith an optical output spectrum of the quantum cascade laser. A combline selector may be included to select and isolate at least one combline from the fiber comb laser, the comb having a comb spacing greaterthan about 250 MHz. A feedback loop locks the output optical frequencyof the quantum cascade laser to the selected comb line. The opticalfrequency of the quantum cascade laser becomes a function of the tunablecomb spacing.

The comb line selector may include one or both of an RF filter and anoptical filter.

The feedback loop may be configured to selectively adjust a temperatureor operating current of the quantum cascade laser.

At least one embodiment includes a high resolution spectroscopy system.The system includes a fiber comb laser having a repetition rate greaterthan about 1 GHz. An optical sub-system may be disposed downstream fromthe fiber comb laser, and configured to optically resolve the individualcomb lines from the comb system. The optical sub-system may include: atleast one or both of a diffraction grating and a VIPA, and a one or twodimensional detector array. The individual elements of the detectorarray may be spaced in such a way that each element is sensitive to anoptical frequency band approximately equal to the comb line spacing.

The high resolution spectroscopy system may include a fiber frequencycomb system having tunable comb spacing, tunable carrier envelope offsetfrequency, or both.

The high resolution spectroscopy system may include: at least onereference laser and at least one detector configured for measuring theinstantaneous optical frequencies of the comb system.

At least one embodiment includes a low phase noise micro-wave source.The system includes: a fiber comb laser with a comb spacing greater thanabout 1 GHz; a reference laser; a first detector to measure a firstmicro-wave beat signal (S1) between a line of the comb laser and theoptical reference laser. The system also includes a sub-system tomeasure a carrier envelope offset frequency of the fiber comb laser, thesub-system receiving an output from the comb laser, and producing as anoutput signal (CEO) representative of the carrier envelope offsetfrequency. A mixer receives the first micro-wave beat signal and theoutput signal (CEO) and generates a second beat signal (S2). A combcontroller controls the comb laser, for example the repetition rate andcarrier phase. The comb controller also receives the second beat signal,and a portion of the comb controller is configured to phase lock thesecond beat signal to a micro-wave reference via modulation of the comblaser spacing. A second detector output provides a low phase noisemicro-wave output signal.

Some embodiments of a mode locked waveguide laser system and/or a lowphase noise micro-wave source may include additional electronic feedbackcircuit(s) configured to stabilize the comb laser output power.

Some embodiments of a mode locked waveguide laser system and/or a lowphase noise micro-wave source may include a highly rare earth doped gainfiber.

In some embodiments a mode locked waveguide laser may be configured as aring laser, or with a Fabry-Perot cavity.

Thus, while only certain embodiments have been specifically describedherein, it will be apparent that numerous modifications may be madethereto without departing from the spirit and scope of the invention.Further, acronyms are used merely to enhance the readability of thespecification and claims. It should be noted that these acronyms are notintended to lessen the generality of the terms used and they should notbe construed to restrict the scope of the claims to the embodimentsdescribed therein.

What is claimed is:
 1. A mode locked waveguide laser system, comprising:a mode locked waveguide laser comprising a laser cavity having awaveguide, said mode locked waveguide laser system operably arranged asa frequency comb laser providing tunable comb spacing, tunable carrierenvelope offset frequency, or both; an intra-cavity beam emitted fromsaid waveguide; and a dispersion control unit (DCU) disposed in saidcavity and in an optical path of said intra-cavity beam, wherein saidlaser cavity is configured in such a way that said intra-cavity beam isredirected to said waveguide after traversing at least said DCU, saidDCU comprising a single optical element operably arranged to induceangular dispersion on said intra-cavity beam, wherein said DCU impartsangular dispersion, and group-velocity dispersion (GVD) to saidintra-cavity beam during propagation in said cavity, and imparts aspatial chirp to said redirected beam, wherein said DCU is capable ofproducing net GVD in a range from a positive value to a negative value.2. The mode locked waveguide laser system according to claim 1, furthercomprising: means for control of the carrier envelope offset frequencyof said mode locked waveguide laser.
 3. The mode locked waveguide lasersystem according to claim 2, wherein said means for carrier envelopeoffset frequency control comprises an optical element and a mechanismfor translating said optical element along an axis.
 4. The mode lockedwaveguide laser system according to claim 1, wherein said mode lockedwaveguide laser is configured to operate at dual wavelengthssimultaneously, and wherein said DCU is configured to provide awavelength separation of said dual wavelengths.
 5. The mode lockedwaveguide laser system according to claim 4, wherein said waveguidelaser comprises a fiber laser, and said waveguide laser is configured toprovide a different repetition rate for each of said dual wavelengths.6. The mode locked waveguide laser system according to claim 1, whereinsaid mode locked waveguide laser is configured such that a temporalbandwidth of said mode locked waveguide laser is limited, at least inpart, by said spatial chirp.
 7. The mode locked waveguide laser systemaccording to claim 1, wherein said DCU is configured to displace atleast one optical element of said DCU so as to adjust said net GVD to avalue within said range.
 8. The mode locked waveguide laser systemaccording to claim 1, wherein said waveguide laser is configured with aring laser or with a Fabry-Perot cavity.
 9. The mode locked waveguidelaser system according to claim 1, wherein said mode locked waveguidelaser is configured to provide a frequency comb at an output thereof,and said mode locked waveguide laser system comprises an electronicfeedback circuit configured to stabilize the output power of saidfrequency comb.
 10. The mode locked waveguide laser system according toclaim 1, wherein said DCU comprises a single diffraction grating and alens system.
 11. The mode locked waveguide laser system according toclaim 1, wherein said DCU comprises a lens system, and further comprisesa prism or grism.
 12. The mode locked waveguide laser system accordingto claim 1, wherein said mode-locked waveguide laser system comprises afiber comb laser having comb spacing of at least about 250 MHz and up toabout 10 GHz, wherein said system configured such that an output saidcomb laser is locked to an optical reference and an microwave beatsignal is generated between said optical reference and said comb laser,wherein said microwave beat signal is subsequently mixed with a signalrepresentative of a carrier envelope offset frequency of the comb laserto generate a secondary beat frequency which is essentially independentof the carrier envelope offset frequency, so as to provide a low phasenoise microwave source with said mode locked waveguide laser system. 13.The mode locked waveguide laser system according to claim 2, whereinsaid means comprises at least one continuous wave (cw) reference laser.14. The mode locked waveguide laser system according to claim 2, whereinsaid means includes a feedback system for pressure and/or temperaturecontrol of intracavity optical element(s).